In recent years rapid progress has been made in the field of stainless steel coloring techniques and various patent specifications have been laid open.
Among the prior publications, Japanese patent laid-open specifications Sho 46-7308, Sho 48-11243 and Sho 49-21339 disclose an epoch-making method for coloring a stainless steel by which better wear resistance and reproducibility can be attained as compared with the conventional art and since the development of this method, colored stainless steels have been commercialized and colored stainless steels are now being produced on a larger scale then previously.
Japanese patent laid-open specification Sho 48-11243 describes, in detail the reproductivity of coloring of a stainless steel.
According to the above Japanese patent laid-open specification, when a stainless steel is immersed in a solution containing chromic and sulphuric acids, the potential between the surface of the stainless steel and a reference electrode varies as shown by the potential-time curves in FIG. 1 and FIG. 2, and the potential variation rate in respect to time shows its lowest value just before a film which assures coloring is formed on the stainless steel surface, while the potential shows its minimum value in case of a saturated calomel electrode (FIG. 1) and its maximum value in case of a platinum electrode (FIG. 2). These potential values are called as "an inflexion point potential (A)", after which the coloring starts, and as the coloring progresses as blue-gold-red-green, the potential raises or lowers as shown in FIG. 1 or FIG. 2. Now taking the potential at which a desired color is produced as the coloring potential (B), the potential difference (A-B) between the inflexion point potential (A) and the coloring potential (B) takes a constant value in case of a given combination of the steel material to be colored and the coloring solution. Therefore, this prior art aims to obtain a single color constantly by predetermined the potential difference (A-B) for a desired color and by stopping the coloring when a potential difference (A'-B') shows a value equal to the predetermined difference (A-B).
However, the present inventors have conducted experiments in which about a dozen of stainless steel grades were colored according to the method of the above Japanese patent by immersing SUS 304 stainless steel sheets of BA (bright annealing) finish (surface finish as specified by JIS G 4305) in an aqueous solution containing 300 g/l of chromic anhydride and 500 g/l of sulfuric acid at 75.degree. C. and maintaining the potential difference between the inflexion point potential and the coloring potential constant at 13 mv (standard potential for red coloring), but the results have revealed that the obtained colored films vary in many color tones ranging gold, red, purple, dark blue, and then intermediate colors as shown in FIG. 4. Thus constant reproductivity of a colored film on a stainless steel can not be attained by the method disclosed by Japanese patent laid-open specification Sho 48-11243.
Further, the prior art for coloring a stainless steel has defects that when stainless steel lots having different production histories, such as, production lot and time are subjected to coloring treatments under the same conditions, different colored films are formed depending on the production histories or in worse cases only dull colors can be obtained.
The present inventors have tried to color stainless steel lots having different production histories according to the disclosure of Japanese patent laid-open specification Sho 48-11243 and have found that the color variation is considerable particularly in the blue and gold colors, meanwhile the variation is not so large in red and green colors which require a long time period of immersion. Thus it can be concluded that the surface condition of the steel sheets varies depending on their production history.
Still further, the prior art for coloring a stainless steel has the defect that an attractive color can be developed when the method is applied to nickel-chromium stainless steels, but the attractive color cannot be obtained, the obtained color is limited to brown or dark brown only and a desired color is not obtained when the method is applied to chromium-straight stainless steels. Therefore, up to now, only costly nickel-chromium stainless steels, such as, SUS 304 have been limitedly used for coloring, and this fact has users to be of the opinion that a colored stainless steel is a highly costly material, and application of a colored stainless steel has been greatly limited.